The present invention is directed to a process for the production of phosphoric acid by the wet process. The invention is directed to the production of phosphoric acid by the calcium sulfate hemihydrate or simply the hemihydrate process. The present invention is directed to the process in which the control of reactant concentrations is improved, a concentrated phosphoric acid (about 30% to about 55% P.sub.2 O.sub.5) is produced, a reduction in sulfuric acid usage is realized and a substantial reduction in electrical energy consumption is also realized.
Phosphoric acid has been prepared by the wet proess for many years. The wet process involves the reaction of phosphatic solid materials, hereinafter termed phosphte rock, with sulfuric acid. A slurry comprising calcium sulfate, monocalcium phosphate, phosphoric acid and sulfuric acid is the usual reaction media. The names of the three processes for the production of phosphoric acid by the wet process are based on the by-product calcium sulfate produced; namely, the gypsum or dihydrate process, the hemihydrate process, and the anhydrite process. The type of by-product is dependent upon the temperature of the system and the P.sub.2 O.sub.5 concentration of the liquid phase of the slurry. Other factors such as fluorine concentration, alumina concentration, and sulfuric acid concentration play a less important role.
Gypsum, CaSO.sub.4 . 2H.sub.2 O, is the by-product formed when the wet process is run at a temperature of 90.degree. C. or less and a P.sub.2 O.sub.5 concentration of about 30% in the liquid portion of the slurry. Increasing the temperature to about 80.degree.-120.degree. C. and the P.sub.2 O.sub.5 concentration to about 40% in the liquid phase will yield hemihydrate, CaSO.sub.4 . 1/2H.sub.2 O. Adjusting the temperature and the concentraions, for instance, to 75.degree. C. and 40% P.sub.2 O.sub.5 results in a mixture of gypsum and hemihydrate which is very unstable. An unstable system such as this causes trouble during filtration due to the hardening or setting up of the gypsum-hemihydrate solid on the filter. Care must be exercised in maintaining the proper temperature and P.sub.2 O.sub.5 concentrations in the process being run in order to avoid such problems. CaSO.sub.4 anhydrite is produced at temperatures of about 130.degree. C. and P.sub.2 O.sub.5 concentrations greater than 30%. This latter process is most difficult to run due to severe corrosion at the higher temperatures and the instability of the anhydrite during processing.
Several problems are inherent in the production of phosphoric acids by the wet process. The degree to which these problems affect the three process will vary due to the different operating conditions employed. Several problems which affect recovery and/or processing of the phosphate rock during the production of phosphoric acid are discussed below.
Phosphate values can be lost during processing of phosphate rock by several different mechanisms. The first consists of the coating of the phosphate rock with calcium sulfate. This impeeds and/or inhibits the recovery of the phosphate values from the rock, hence resulting in very low yields. The second consists of substitution of calcium phosphate within the calcium sulfate lattice. The substituted phosphate values cannot be recovered by washing during the separation stage and hence pass to waste. This again results in poor recovery from the phosphate rock. The third problem involves the rapid precipitation or crystallization of many very small crystals of calcium sulfate. This lads to very poor filtration and filterability. The conditions which are employed in the three wet processes are listed and their effects on the recovery of P.sub.2 O.sub.5 from the rock.
As the P.sub.2 O.sub.5 concentration of the liquid portion of the reaction slurry increases (about 28% P.sub.2 O.sub.5 for the dihydrate process; about 40% P.sub.2 O.sub.5 for the hemihydrate process and about 50% P.sub.2 O.sub.5 for the anhydrite process), there is a great tendency to increase the substitution of calcium phosphate within the calcium sulfate crystal lattice. This results from the increase in HPO.sub.4.sup.-2 concentration in the liquid portion of the slurry. In the same manner the increase in the P.sub.2 O.sub.5 concentration of the liquid portion of the slurry tends to increase the viscosity of the reaction media and hence also tends to increase the amount of substitution of the phosphate within the calcium sulfate crystal structure due to reduced diffusion of the HPO.sub.4.sup.-2 species within the slurry. If, however, the temperature is increased, as occurs from going from the dihydrate process to the hemihydrate process, the viscosity of the reaction media is lower and hence the degree of substitution of the calcium phosphate within the calcium sulfate crystal structure is decreased. It must be recognized, however, that there are temperature limitations which must be observed for the process under consideration.
Increasing the sulfate concentration in the liquid phase of the slurry results in a decrease in the calcium ion concentration, thus tending to decrease the amount of substitution of calcium phosphate within the sulfate crystal lattice. However, care must be exercised not to increase the sulfate concentration to such an extent that the dissolution or the recovery of phosphate values from phosphate rock is impeeded by the coating of the rock with a layer of calcium sulfate. Excess sulfate concentration in the presence of high localized concentrations of calcium ions results in the precipitation of many very small crystals of calcium sulfate, resulting in a slurry difficult to filter. Thus the sulfate concentration can act both to increase the recovery of phosphate from the phosphate rock, or it can result in reduced recoveries of phosphate from the phosphate rock with attendant reduced filtration rates.
An increase in solids in the slurry will tend, in general, to increase crystal growth of the calcium sulfate formed by the reaction of calcium ions with sulfate ions. This will tend to result in larger crystals which will be more easily filterable and washable. In general, the variation of the solids content results only in very small variations in the degree of substitution of calcium phosphate within the calcium sulfate crystal lattice. In addition, it is imperative not to increase the solids to such an extent that the viscosity of the slurry is increased to such an extent that mixing becomes very difficult and localized supersaturation occurs.
Thorough mixing is very desirable whether running the dihydrate, the hemihydrate or the anhydrite process. Good mixing will decrease the localized high concentration of the reactants; namely, the calcium phosphate and the sulfuric acid. Decreasing such localized concentrations, results in a lowering of the substitution losses, a lowering of losses due to coating the rock and an improvement in the crystallization conditions.
Thus, it is observed that a change of one variable may favorably affect the recovery of P.sub.2 O.sub.5 from phosphate rock employing one of the wet process methods and it may be detrimental to the recovery of P.sub.2 O.sub.5 employing a different process. Therefore it is necessary to choose the combination of process variables which will result in the best recovery of P.sub.2 O.sub.5 from the phosphate rock along with acceptable filterability of the resulting slurry for the process at hand.
The recovery of the phosphate values from the phosphate rock can be greatly increased if the agitation or mixing is maintained at a high level. Previous workers in the field have directed their energy to achieve maximum mixing in the wet process. As a result of this activity, today there are one vessel and multi- vessel systems in use for the production of phosphoric acid by the wet process. The purpose is to achieve maximum mixing so as to increase the recovery of the phosphate values from the phosphate rock and to have the best environment for dissolution of the rock and for crystallization of CaSO.sub.4.
In a one vessel process, the phosphate rock and the sulfuric acid are added to the slurry in one tank. Agitators, in union with baffles, are used to circulate the slurry into which the reactants (phosphate rock and sulfuric acid) are added. To the extent that the localized concentration differences are minimized, the slurry has only one sulfate level. This is undesirable, since the phosphate rock should preferably be dissolved at a low sulfate concentration whereas crystallization should occur at a high sulfate concentration.
A multi-vessel system can be of two types. Two or more compartments or cells can be constructed within one vessel, the compartments being interconnected in series. The reactants are added separately, that is, in different compartments in order to increase the dispersion of said reactant in the slurry prior to reacting with the other reactant. At the last compartment, some slurry is removed from the system for recovery of phosphoric acid; the major portion of the slurry being recycled to the first compartment.
Multi-vessel processes involve the use of two or more vessels connected in series, the reactants are added to the slurry in separate vessels so as to more completely disperse one reactant in the slurry before it is contacted by the later added reactant(s). Again the system is arranged so that a portion of the slurry is recycled from a later reactor back to the first reactor.
The reaction between sulfuric acid and phosphate rock is exothermic. In order to control the temperature of the reaction system, provisions must be made to remove this excess heat. Previously this has been accomplished by (1) blowing air through the slurry or (2) pumping a portion of the slurry to a vessel under vacuum or (3) conducting the operation in a vessel under vacuum.
The use of air as a coolant is not too desirable because it is necessary to scrub large amounts of air exiting the system to remove pollutants, mainly fluorine in the form of hydrogen fluoride or silicon tetrafluoride. The equipment required is quite expensive. When a portion of the hot slurry is removed from the main body of the slurry, and subjected to vacuum, cooling occurs by the evaporation of water (U.S. Pat. No. 2,699,985). The cooled slurry is then added to the main body of the hot slurry and moderates the temperature of the process.
The method of conducting the reaction under vacuum has many desirable features. The cooled slurry is immediately dispersed within the hot slurry and temperature differentials within the slurry areminimized. The slurry is concentrated by the removal of water, and the desired temperature is easily maintained. The above described multi- compartment and multi- vessel systems improved on dispersing the reactants to some extent, however, greater dispersion of the reactants is desirable in order to improve the dispersion of the reactants in a one vessel reactor. Caldwell, U.S. Pat. No. 3,415,889 and 3,939,248 and Bergstrom, U.S. Pat. No. 3,666,143 and 3,917,457 developed a combination reactor-cooler which is fitted with a draft tube. The vessel was maintained under a vacuum while the slurry was circulated within a vessel. Using the draft tube with an agitator it is possible to circulate the slurry at such a flow rate that upwards of 200% of the volume of the slurry is circulated through the draft tube per minute, constantly renewing the surface of the slurry exposed to the vacuum. With this type of circulation, dispersion of the reactants is improved over the conventional one vessel system. In addition to better dispersion of the reactants, the slurry on exposure of the vacuum at the surface is cooled by evaporation of water. The temperature differential within the system is minimized by the rapid flow rate realized. The cooled slurry is immediately mixed with the hot slurry minimizing the localized cooling affect.
Lopker, U.S. Pat. No. 3,522,003 and 3,522,004 describes a two vessel system for the production of phosphoric acid from phosphate rock and sulfuric acid. These processes involve passing a slurry of phosphoric acid and calcium sulfate through a circuit which contains two vessels in series, at least one of which is under vacuum. The vacuum applied to one vessel cools the slurry by evaporation of water. The cooled slurry is then rapidly dispersed within the system minimizing cooling effects and preventing supersaturation of the calcium sulfate due to reduced temperatures. The levels of the slurries within the two vessels are vertically offset.
Sulfuric acid, phosphoric acid, phosphate rock or a mixture of phosphoric acid-phosphate rock can be added to the slurry in different vessels. The reactants are mixed in the vessel and are circulated from one vessel to another. In this way localized high concentrations of the added reactants are minimized. Good recovery of P.sub.2 O.sub.5 values from the rock are realized. Better filtration rates are also obtainable due to the retardation of the formation of excessive number of very small calcium sulfate crystals resulting from supersaturation.
Processes for the production of phosphoric acid by the hemihydrate process are well known in the art. A. V. Slack, in "Phosphoric Acid" Part One, Marcel Dekker, Inc., New York, 1968, describes hemihydrate process. The problems encountered are observed in filtering the hemihydrate slurry and the high degree of substitution of phosphate in the calcium sulfate lattice. Attempts to overcome the deficiency in filtration rate and poor P.sub.2 O.sub.5 recoveries while maintaining the production of phosphoric acid containing about 40% P.sub.2 O.sub.5, resulted in the development of a hemihydrate-dihydrate system. U.S. Pat. No. 3,472,619 and 3,552,918 are representative of the systems of the systems employed.
These patents describe the preparation of phosphoric acid by the hemihydrate process, recovering said phosphoric acid from the solid CaSO.sub.4 . 1/2 H.sub.2 O, recrystallization of CaSO.sub.4 . 1/2H.sub.2 O to CaSO.sub.4 . 2H.sub.2 O, and the recovery of phosphoric acid liberated during the recrystallization of CaSO.sub.4 . 2H.sub.2 O. Apparently, the best of both processes is achieved. High concentration, about 40% P.sub.2 O.sub.5 acid is recovered while low losses in the filter cake are observed as the result of the recrystallization of the CaSo.sub.4 . 1/2 H.sub.2 O.
Fitch (U.S. Pat. No. 3,552,918) describes a process for the production of concentrated phosphoric acid and gypsum including the acidulation of phosphate rock in a first zone in which the resulting slurry contains from about 1% (-2.45% SO.sub.4.sup.=) to about 4.5% (-11% SO.sub.4.sup.=) excess calcium. The slurry produced in the first zone is then transferred to a second zone in which an excess of sulfuric acid is present such that from about 3% to about 6% excess sulfuric acid is present in the slurry. Hemihydrate initially produced is converted to gypsum.
Long (U.S. Pat. No. 3,453,076), Peet (U.S. Pat. No. 2,885,264) and Robinson, (U.S. Pat. No. 3,418,077) described processes for the production of phosphoric acid by the hemihydrate process. No additional recrystallization of the CaSO.sub.4 . 1/2H.sub.2 O is required in these processes. In the Robinson process phosphoric acid containing from about 40% to about 55% P.sub.2 O.sub.5 by weight is produced. This process which comprises in a first stage reacting in the presence of excess calcium ions, phosphate rock with at least nine parts by weight of phosphoric acid for each part of calcium added, said phosphoric acid containing at least 37% by weight P.sub.2 O.sub.5 and 1% to 3% by weight dissolved sulfate whereby the phosphate rock is converted into a slurry comprising monocalcium phosphate, phosphoric acid, and calcium sulfate, the percentage of calcium ion precipitated as calcium sulfate being 10 to 60%, preferably 20-50% by weight of total calcium fed, in a second stage reacting the slurry from the first stage with sulfuric acid whereby phosphoric acid containing at least 40% P.sub.2 O.sub.5 by weight and calcium sulfate hemihydrate is formed, the sulfuric acid being used in an amount 0.5 to 2.0% by weight in excess of that required to convert the calcium content of the phosphate rock fed to the first stage into calcium sulfate, and in the third stage separating the phosphoric acid from the calcium sulfate and washing the crystals. The temperature of the first and second stages being in the range from 80 to 115.degree. C., preferably from 90-110.degree. C.